1. Field of the Invention
This invention relates generally to the field of focusing apparatus and, more particularly, selectively actuatable apparatus for automatically causing a variable focus system to have a lens element positioned at a preselected focal position, and for suppressing firing of a source of artificial illumination.
2. Description of the Prior Art
Ultrasonic ranging systems are well-known for use in controlling operations in photographic apparatus. Essentially, ultrasonic energy is transmitted to a subject and the reflected signals from the subject are received by the system. Characteristics of the transmitted and received signals are compared for purposes of deriving a ranging signal indicative of the subject distance. Such signal may be used for purposes of driving an element of a variable objective lens assembly of the photographic apparatus to a position corresponding to the subject distance, whereby the subject will be in focus at a focal plane in the photographic apparatus.
An example of such a system is disclosed in U.S. Pat. No. 4,199,246, entitled "Ultrasonic Ranging System for a Camera", by J. Muggli, assigned in common herewith. This patent describes use of an ultrasonic transducer for transmitting a relatively short burst of ultrasonic frequencies, and a synchronized receiver for processing an echo signal produced by the transducer upon receipt of the echo after a predetermined time interval. This receiver produces a ranging signal having a characteristic related to the distance of the nearest subject being photographed. In response to the ranging signal, a lens element of a variable focus lens system is moved to the appropriate focal position corresponding to the subject range.
Despite several significant advantages of ultrasonic ranging systems and the widespread commercial acceptance thereof, certain photographic conditions exist wherein there are potential for problems. For example, focusing difficulties arise when the object to be focused is not the nearest object in the scene, for example, when it is situated beyond an acoustically opaque, but visually transparent material, such as a window. This is because the rangefinder will range on the window rather than the object beyond the window. Obviously, the desired subject will be misfocused at the focal plane.
Towards the end of correcting such problems efforts have been undertaken to provide these types of automatic focusing systems with an override system enabling independent manual control of the focusing system. Examples of such efforts are described in U.S. Pat. Nos. 4,149,792, 4,153,903 and 4,160,590, all commonly assigned herewith.
Thus, the foregoing patents disclose a manual control system for focusing in addition to an automatic focusing system. In such systems a manual focus wheel is operable for achieving the manual focusing when the automatic rangefinder is disabled. Disabling of the automatic focus control system may be effected by rendering the automatic focus control insensitive to the output signal from the rangefinder, by disabling the rangefinder itself or by any combination thereof.
In U.S. Pat. No., 4,149,792, a spring force biasing means is operative in the manual focusing mode for urging the lens from a position outside its normal focusing range to a position inside the normal focusing range to provide correct operation when the camera is later returned to the automatic mode. This latter positioning is therefore done to facilitate the automatic focusing mode and, consequently, does not directly overcome the above-noted problem associated with misfocusing of automatic rangefinders. Commonly assigned U.S. Patent No. 4,160,590 discloses a lens movement limiting stop that limits manual movement of a lens to within its normal focusing range. This patent, like other prior art, requires focusing control by the operator when faced with the problem of focusing upon objects located behind acoustically opaque and visually transparent material.